Photovoltaic systems, abbreviated in the following as PV systems, are used to convert sunlight into electrical energy. Usually, a plurality of photovoltaic modules, abbreviated in the following as PV modules, each of which represents an interconnection of a plurality of solar cells, are electrically connected together as a photovoltaic generator. The photovoltaic generator (PV generator) is often connected to a remotely mounted inverter, which is used to convert the direct current (DC) supplied by the PV generator into alternating current that is suited for feeding into a public or private (isolated operation) power supply network.
Thereby the PV modules are most times connected in series in such a way that the DC power cables between the PV generator and the inverter are exposed to voltages in the range of distinctly more than 100 volts. A voltage of this magnitude is useful for efficiency reasons, to keep, among other things, ohmic loss in the lines tolerably small, without a need for selecting a large cable cross-section. When light strikes the PV modules there is, however, in case of damage, e.g., in case of a fire, or during installation and maintenance the risk of an electric shock that might be dangerous to life, due to high voltage. Without further protective measures, the danger to life in case of direct contact or indirect contact can be banned only if electricity generation by the PV modules is prevented, for example, by a darkening of the PV modules. However, this is difficult to be implemented for large PV systems or in case of fire.
Another problem with PV systems is the danger of an electric arc formation, due to the high DC voltage in conjunction with high DC currents flowing from the PV modules to the inverter. An electric arc may occur, for example, during maintenance (when disconnecting a current-carrying wire), in case of contact degradation at screwing connectors or plug-in connectors, at poorly soldered joints or bad screwing connectors or in case of a damaged cable insulation. Extinction of an arc that has occurred is usually only possible by a drastic reduction of the current flowing through the arc.
To prevent the occurrence of hazardous voltages in case of fire or maintenance during operation of a PV system, it is known to arrange switching devices, such as air gap switches or semiconductor switches, in close proximity to the PV modules, for example, in a junction box of the PV module, the switching devices switching the connection lines between the PV modules and the inverter into a de-energized state when being switched via central lines by the inverter or another control center. This can be done by an interruption of the connection lines by the switching devices or by shorting the PV modules, as for example disclosed in the Photon magazine, May 2005 issue, pp. 75-77. As disclosed in the document DE 10 2005 018 173 A1, the connection lines between the PV generator and the inverter can also be de-energized by a single switching device arranged at the PV generator.
For the transmission of control signals to the switching devices, additionally installed cables are used, which, however, in particular with large PV systems, is associated with increased installation costs. As an alternative, it is known from the publication DE 10 2006 060 815 A1 to apply the control signals as signals at a high frequency by means of a transformer to the DC cables for power transmission and to transfer these signals via these cables. For this purpose, the switching devices are provided with a control unit that decodes the control signals transmitted at a high frequency, and that controls the switching operation. This solution is also expensive, since for the application of the control signals a material-intensive transformer is required, owing to the high currents on the DC lines.